frost buildup in GE Arctica refrigerator

I had heavy frost build up in my GE PSS29NGNAWW freezer section. I have replaced the maun board and the defrost thermostat. I have measured the continuity for the good one and the bad one, they both measure almost zero ohm in room temperature Is my old thermostat good or bad? I thought it should have continuity at cold temp and none when gets warmer? Am I correct? But my problem still exist, The evaporator coils inside the freezer back panel buildup frost? I can turn on the heather manually from diagnostic menu (1 - 4).

thermostat is probably good. On your model, the evap thermistor and board regulate defrost. The defrost thermostat in this model is included as a safety measure only. It closes at 110 f and opens at 140f. As a general rule, most (but contrary to popular belief, not all!) GE models that use thermistors have this type of thermostat /limit.

More than likely you have a bad evap thermistor.. test/replace it.. if it tests ok or problem continues after replacement , then replace board

Sounds like a classic case of a bad evaporator thermistor (clipped to the evaporator) causing incomplete defrosts. It gets water in it which raises its resistance and makes the muthaboard think it should stop defrosting too early. Check the resistance of that thermistor with it dunked in a glass of ice water, should get around 16 k-ohms. Measure the resistance from its wire harness connector back on the muthaboard.

I have ran the thermistor test (0-7) too and they all pass. should I trust this test response?

I've never seen these thermistors fail by going open or short, which is all the self test checks. Basically useless. They'll usually go out of spec. And a thermistor needs to have a load to check it, hence the ice water test.

I measured the thermister right on the evaporator when the freezer was cold. It is 23K-ohm. then I heated up with hair dryer, and went don to 8K-ohm, and started to rise slowly. so is this a good test or I have to isolate it and do the ice water test

also, you said water gets in the thermister and raises the resistance, but the higher the resistance means lower temp on the evaporator, and that should acttualy make the defrost heater stay on longer not make it stop? am I correct?

I measured the thermister right on the evaporator when the freezer was cold. It is 23K-ohm. then I heated up with hair dryer, and went don to 8K-ohm, and started to rise slowly. so is this a good test or I have to isolate it and do the ice water test

The specifications for the thermistor are given as resistance (or voltage drop) at a specific temperature of the thermistor. Without knowing the exact temperature of the thermistor at the time of the measurement, the resistance or voltage drop measurements are meaningless.